As the recent trend of cutting tools, the temperature of the cutting edge of the tool is prone to higher temperature due to the fact that: there is a demand for dry working dispensable of a cutting oil agent from the standpoint of global environmental protection; the variety of workpiece material has become immense; the cutting speed has become higher in order to further improve the working efficiency; and the like. Moreover, since the cutting work in recent years frequently requires deep cutting and more feeding, the demand in property for the tool material is becoming more severe.
With regard to the demand on the property for tool materials, improvement in the wear resistance and defect resistance related to the lifetime of a cutting tool, as well as stability of the coat film formed on the base material at high temperature (anti-oxidation property and adherence of the coat film), have become more critical.
To improve the wear resistance and surface protection function, the surface of a cutting tool formed of a hard base material such as of WC based cemented carbide, cermet, high-speed steel as well as wear-resistant tools is coated. The main stream of such coating is effected by chemical vapor deposition (CVD). Particularly, MT (moderate temperature)-CVD for coating at a relatively middle to low temperature, or an HT (high temperature)-CVD for coating at a high temperature greater than or equal to 1000° C. is widely employed.
Japanese Patent Laying-Open No. 2008-087150 (PTL 1) can be cited as a publication disclosing prior art employing MT-CVD. According to PTL 1, raw material gas including CH3CN and chain hydrocarbon with the carbon number 2-20 are introduced as the excessive carbon source to apply a coat of TiCN layer by MT-CVD. A TiCN layer formed in such a manner has a crystal structure of a columnar structure with a great amount of carbon. Then, the surface of the TiCN layer is coated with an aluminium oxide layer that is superior in anti-oxidation.
Japanese Patent Laying-Open No. 63-195268 (PTL 2) can be cited as a publication disclosing prior art employing HT-CVD. According to PTL 2, CH4, N2, and TiCl4 are introduced as the main raw material to apply a Ti carbonitroxide layer on the surface of the base material by HT-CVD.